Mammals, especially higher vertebrates including human, have developed highly complex immune systems that use multiple mechanisms and effectors to detect, destroy, or at least contain foreign pathogens as well as diseased or stressed autologous cells. These diseased cells may have been infected by virus or bacteria, or have become cancerous.
One of the mechanisms for the immune system to recognize and eliminate diseased host cells and invading intracellular microorganisms (e.g., viruses, bacteria or parasites) is through cell-mediated cytotoxicity, which can be carried out by a number of leukocytes and proteins. These potentially cytotoxic effectors include: from the lymphoid lineage—Natural Killer (NK) cells and cytotoxic T lymphocytes (CTLs); and from the myeloid lineage—macrophages, neutrophils and eosinophils.
An important way for the immune system to unleash cell-mediated cytotoxicity relies on antibodies. Over the past decade, monoclonal antibodies (mAbs) that target tumor-specific cell-surface proteins have become a popular therapeutic approach against cancers. Several mAbs have entered routine clinical practice including rituximab (Rituxan, Mabthera), trastuzumab (Herceptin) and cetuximab (Erbitux). The popularity of mAbs is a result of their bifunctional nature. One end of an antibody (Fab) can be made exquisitely specific to a particular tumor protein without altering the other end (Fc) which recruits a variety of effector cells and proteins that kill the tumor cell.
Specifically, after recognizing and binding antigens on the surface of a target cell first, antibodies act as an adapter and proceed to activate the cytotoxic capability of immune effector cells through a second binding with certain receptors on those effector cells. This is called the antibody-dependent cellular cytotoxicity (ADCC). For example, in the context of innate immunity against cancer, ADCC is primarily mediated by natural killer (NK) cells (and, to a lesser extent, neutrophils, monocytes and macrophages) that express a relatively low-affinity Fc receptor (FcγRIIIa, also known as CD16a) that is only activated upon binding the Fc (constant) portions of antibodies coating a multivalent antigen on a target diseased cell (e.g., a tumor cell). This binding triggers the release of cytotoxic granules like perforin and granzyme (as well as many cytokines including IFN-γ), leading to the lysis of the target cell. The importance of the ADCC response has been shown both in vitro as well as in animal studies. Moreover, several clinical studies have shown that patients carrying a lower affinity variant of CD16 (F158) have worse clinical outcomes.
However, ADCC efficacy, as primarily mediated by endogenous natural killer (NK) cells, is limited in the body due to a number of physiological as well as pathological reasons as explained below (to the extent that endogenous Cytotoxic T lymphocytes participate in tumor clearance at all, their efficacy has also been found to be very limited and lacking).
First, most cells involved in the ADCC response such as macrophages and neutrophils do not tend to proliferate when they are activated. NK cells also have limited proliferation potential in response to activation, and they also rapidly die off Therefore, natural ADCC response in the body risks being overwhelmed by disease progression (e.g., viral infection, cancer) even if the ADCC effectors recognize antibodies coating diseased cells.
Second, many of the ADCC effector cells also express inhibitory receptors that dampen their immune responsiveness, thereby instituting a system of balances and checks. These receptors include inhibitory KIRs (killer immunoglobulin-like receptors) for CD56low NK cells, FcγRIIb on monocytes and B cells, and CTLA-4 (CD152) and PD-1 (Programmed-Death-1, CD279) for T cells. Cancer cells and viruses counteract body's ADCC-based defense system by abnormally amplifying such inhibitory pathways.
Third, the main Fc receptor on ADCC effector cells, FcγRIIIa, has a relatively low affinity (Kd≈10−6 M) for antibodies—even the V158 variant of the receptor has only a two-fold higher affinity compared to the ineffective F158 form of the receptor. This is one mechanism through which cancer cells become resistant to some therapeutic monoclonal antibodies (mAbs) once the density of the cell surface targets fall below a certain level.
In view of its natural limitations in proliferation and affinity as well as further depression through inhibitory Fc receptors in the setting of a disease, such as cancer or other diseases, the body's ADCC function has great potential that is never fully realized. Therefore, synthetic biology represents a novel and highly desirable approach to unleash the full potential of ADCC activity in the prevention and treatment of human diseases.